This invention relates to a system for detecting an alternating voltage and/or current, and more particularly to a system for extracting some instantaneous values of an alternating voltage or current as sampled values to determine the amplitude of the alternating voltage or current from these sampled values.
As the system for detecting an alternating voltage and/or current of this kind, there are known the rectifier addition system and the productive addition system for determining the amplitude of a voltage and/or current in digital protective relays, etc. (see Protective Relay Engineering of the University Lecture of Institute of Electrical Engineers issued by IEEJ (Institute of Electrical Engineers of Japan), Jul. 20, 1981).
The rectifier addition system is the system for applying rectifier addition operation to a fundamental wave of an alternating voltage to convert it to a d.c. (direct current) voltage with the same precision as in the case of a sort of polyphase rectification. The ripple components correspond to errors by the sampling phase. Within this type of system there are variation such as the system in which the sampling period (time interval) is 90 degrees and two sampled values are used to perform the rectifier addition operation. Also known is the system in which the sampling period (time interval) is 30 degrees and six sampled values are used to perform the rectifier addition operation, and the like.
On the other hand, the productive addition system is the system in which the sampling period (time interval) is 90 degrees and two sampled values are used to perform productive addition operation of a fundamental wave. With this system, if there is no error in the sampled values, there occurs no error in the amplitude determined.
However, regardless of the type of rectifier addition system employed, since the operational equation itself is the approximate equation, there occurs an error dependent upon the phase of sampling.
With the productive addition system, no error occurs from a theoretical point of view, but since the operation is performed by using only two sampled values, there is the possibility that a large error may occur due to noise, etc.